The general aims of this research are to evaluate the effect of photocoagulation on various types of diabetic retinopathy, to elucidate the natural history of diabetic retinopathy and to work in a collaborative manner for determining the effect of various types of photocoagulation and modes of treatment on diabetic retinopathy and in preserving vision. To achieve these objectives, a collaborative program with a precise protocol has been established. At participating institutions, patients with specific forms of diabetic retinopathy will complete detailed ophthalmic and general medical baseline studies. Retinal photocoagulation, with the xenon photocoagulator and/or the argon laser photocoagulator, will then be given (to one eye of each patient) according to a centralized statistically controlled pattern. Thereafter, precise follow-up studies will be carried out at stated intervals over a 5 year period.